In order to determine the conformity of shape and composition of normally identical manufactured items and to identify unseen objects such as objectional items in luggage at an airport, it is often necessary to obtain more information concerning distribution of matter within the objects examined than is provided by conventional radiographs. The subject invention provides a means of obtaining this information for objects transported on a conveyor belt of similar rectilinear transport means. For example, conventional radiographic devices reveal the presence of voids in castings, but not their location. Similarly, while such devices reveal a pistol in luggage, they are incapable of revealing several types of bombs. This invention provides information which enables identification of a far wider variety of imperfections and items in opaque objects. This identification can be by visual inspection of a reconstructed image or by automatic analysis of a signal produced by the examined object having a detected deviation from a predetermined standard.
In prior devices, information concerning the distribution of matter, usually defects, has typicaly been obtained by stereo radiography. Using either an x- or gamma-ray source, film shadowgraphs are obtained for two orientations of the source and film relative to the object. These images are superimposed and the locations of the defects are obtained by noting the displacements of the defect images of known features. The determination may be made visually, with the aid of a stereo viewer, or analytically, by measuring displacements on the two shadowgraphs.
An extension of stereo radiography, termed longitudinal tomography, has been employed to image single planes within an object. Longitudinal refers to the orientation of the imaged plane relative to the long axis of the human body, which is the principal object examined. U.S. Pat. No. 2,400,516 to Kieffer teaches such a device. Also, "Tomosynthesis: A Three Dimensional Radiographic Imaging Technique: by D. G. Grant in the January 1972 issue of IEEE Transactions on Biomedical Engineering discusses such devices. A single film plate and, usually, an x-ray source are synchronously rotated about an object during exposure of the film such that one plane remains in focus while neightboring planes are blurred.
Recently, considerable advances have been made in examining opaque objects by computerized transverse axial tomography, referred hereinafter as CTAT. Transverse axial refers to the orientation of the imaged plane relative to the long axis of the human body. CTAT replaces film when a radiation-sensitive detector or detector array; images are constructed by a computer and, most often, diplayed on a cathode-ray tube. The source and detector are synchronously rotated through a semicircle about the object being examined. In presently available CTAT commercial equipment, a single detector and pencil-beam x-ray source are scanned across the object under examination for each orientation. Such devices are disclosed in U.S. Pat. No. 3,778,614 to Hounsfield. Also, the October 1975 issue of Scientific American has an article on CTAT and longitudinal tomography: "Image Reconstruction from Projections" by R. Gordon et al.